Implantable medical leads include electrodes at a distal end in order to provide electrical stimulation to tissue at a target site within the body and/or to provide sensing of physiological signals at the target site. The implantable medical leads have a proximal end that is coupled to an implantable medical device (IMD) that performs the electrical stimulation and/or physiological sensing. Electrical conductors extend through the implantable medical leads from the IMD, which is positioned at a convenient implantation site, to the electrodes located at the target site.
During procedures such as a magnetic resonance imaging (MRI) scan where a level of radio frequency electromagnetic energy much greater than in normal ambient conditions is present, an electrical current is induced onto the electrical conductors of the lead. This electrical current passes through the electrode to generate heating of the electrode to tissue interface, and this heating can be potentially dangerous to the patient having the implantable medical system that includes the IMD and one or more leads. Various techniques may be used to reduce the degree of heating. One example is to include a shield within the lead that surrounds the electrical conductors. Other examples include increasing the impedance of the electrical conductors via chokes and the like.
These techniques have been proven effective for implantable medical systems where the lead is routed well below the surface of the body of the patient. However, in some cases, the lead may be routed near the surface, such as for peripheral nerve stimulation therapy. In such a case, the shallow depth of the lead within the body results in exposure to higher levels of the RF energy. These higher levels may exceed the capabilities of a lead to prevent excessive heating at the electrode to tissue interface for a lead designed originally to lessen the effects of RF energy for a deeper type of implantation.